Iron base alloys and articles made therefrom



United States Patent 3,047,484 IRON BASE ALLOYS AND ARTKCLES MADE THEREFROM John T. Stacy and Henry A. Saller, Columbus, Ohio, as-

signors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Mar. 30, 1955, Ser. No. 498,119

- 5 Claims. ((Il. 204-1932) This invention deals with iron base alloys and in particular with chromiumand aluminum-containing iron base alloys and articles made therefrom.

The alloys forming the subject of this invention are primarily intended to be used for structural elements, electrical heating elements and for fuel element jackets for neutronic reactors; jackets made of the alloys of this invention are particularly well qualified for the fuel elements in reactors of supersonic airplanes.

For one or the other purposes just listed iron alloys have been used heretofore which contain from 5 to 30% by weight of chromium and from 3 to by weight of aluminum. These alloys have a fair resistance to oxidation, and the higher the contents of aluminum and chromium are, the higher is the resistance. However, with increasing aluminum and chromium contents the workability of these known alloys decreases. Moreover, the ternary-chromium-aluminum-iron alloys, at temperatures above about 1100 '1 lose strength rapidly with increasing temperature. It has been tried to overcome this drawback by adding a small amount of carbon to the ternary alloy. The high-temperature strength was actually improved bythis addition; however, the oxidation resistance was markedly impaired thereby.

3,047,484 Patented July 31, 1962 melted four to six times to obtain homogeneity. For test- 7 ing the oxidation resistance and workability of the alloys thus produced the alloys were cast into buttons of 50 grams each. For the rupture tests rectangular blocks of 300 grams each were prepared.

These blocks and buttons were then subjected to an annealing step for homogenization which consisted of heating between 1800 and 2200 F. for about 18 to 20 hours. The alloys were then rolled at between 2200 and 2500" F. to 0.070-inch thick strips. By this rolling procedure a total reduction in area of 80 to 85% and in each pass a reduction between 5 and 20% was effected; reheating was carried out after each pass. All alloys of this invention showed good workability and could be rolled without difiiculty.

For testing the oxidation resistance of the alloys at elevated temperature, specimens were cut from the rolled strips; the specimens had dimensions of 1" x /2 x 0.055". The surfaces of the specimens were finished on a 400-grit emery paper. A small end section of each specimen was cut off in order to have a comparison between the thickness of the oxygen-exposed specimen and that of the original specimen. Each sample, before testing, was weighed It is an object of this invention to provide iron-base base alloy which is superior to the ternary-aluminumchromium-iron alloys in regard to strength at high temperatures.

It is finally also an object of this invention to provide an iron-base alloy which has a comparatively low thermalneutron-absorption cross section.

These objects are accomplished by adding to the ternary aluminum-irou alloy tantalum and/or niobium. While ranges of from 15 to 30% of chromium, from 5 to 6.5% of aluminum and from 1 to 10% of at least one of the metals niobium and tantalum give very satisfactory results, the preferred ranges are from 2.5 to 10% for the niobium and from 5 to 10% for the tantalum. (All percentages in percentages.)

The addition of nadium, the other element pert-aining to the same pot? the periodic system of the elements as niobium ail'dtantalum, did not bring about the same favorable results. j The alloy containing vanadium instead of niob tantalum showed a highly unsatisfactory resistafice tp oxidation at elevated temperature. Likewise, berylliuin,;molybdenum, titanium, tungsten or zirconium, when used as additives instead of niobium and/or tantalum, impaired the oxidation resistance of the ternary iron alloy-considerably.

The alloys of this invention can be prepared by any method known to those skilled in the art. The components may either be melted together in the desired proportions or the additives of this invention may be added to a master alloy, namely, the ternary chromium-aluminum-iron alloy. The method that was usually applied for the studies reported in this specification comprised arc-melting of the components in a water-cooled copper crucible using a helium atmosphere and tungsten electrodes. After melting, the alloys were inverted and rethis specification are given as weight and measured and then cleaned in acetone.

Two kinds of tests were carried out in order to determine the resistance to oxidation; for both the same type of specimens were used. One test was the constanttemperature test in which the specimen was exposed to air at a constant temperature of 2200 F. for hours in a horizontal open-tube furnace and then weighed together with all the oxide that had spalled.

For the other test, the cycling test, the specimen was cycled between a zone having a temperature of 850 F. and a zone of 2200 F., both in an air atmosphere. The specimen was kept in each zone for 15 minutes and 200 cycles were carried out; for each test a total of 100 hours was thus needed. After the cycling the specimen was mounted along with the cut-off control end portion. The specimen was polished and measured optically to obtain the oxide penetration value. One-half of the decrease in thickness of the specimen was considered the penetration value. I

Furthermore, the alloys were subjected to high temperature strength tests in which shorttime stress rupture I and creep rupture were measured. For both purposes hot-rolled strips 0.070 inch thick were ground on the surfaces and machined to form specimens 10 /2 inches long, 4 inch wide and 0.060 inch thick. The gauge length waltwo inches and the gauge width 0.5 inch.

The specimens were brought to 2200 F. in an air atmosphere in an electrically heated furnace. For the short-time rupture tests a tensile load was applied to the specimen after 15 minutes of heating by means of a calibrated air cylinder; the load was progressively increased until sustained elongation was obtained. The time needed from the start of sustained elongation until occurrence of rupture was recorded as the rupture time.

For measuring the creep rupture, the specimen was also inserted in an electrically heated furnace; the temperature was controlled so that at the center of the gauge length it varied by about i4 F. The load was applied to the specimen by direct loading with a dead weight after the specimen had reached the test temperature. The creep was measured periodically with an external dial gauge. The data thus obtained were converted to percentages and time-deformation curves were plotted therefrom. From these curves the minimum creep rates were calculated. The times of rupture were recorded automatically, and the total elongation values were measured after the rupture.

The hardness of the alloys was also ascertained. For this purpose specimens x A" were cut from the hot 3 rolled 0.070-inch thick strip and heat-treated for 18 to 20 hours at 1800 to 2200 F. The thus treated specimens were then tested for their Rockwell A hardness.

The results obtained in these above-described experiments of the alloys according to this invention and some 5 and the balance of said alloy being iron. other alloys also tested are compiled in the following 3. A acket for a fuel element of a neutronic reactor table. consisting of an alloy consisting of 25% by weight of N Omlnal Composition, percent by Constant- Oycling- Short-time rupture Hardness of weight temp. oxidaoxidation at 2200 F.- Total Minimum h0t-r0lled tion-weight penetration, elongation, creep rate, alloy at room gain, iL/infi inch percent in percent hr. temp.,Rock- Cr Al Nb Ta Others (100 hrs) Stress, Rupture 2in. well A p.s.i. time Inconel 0. 0274 310 Stainless Steel 0. 0274 Minus sign shows growth greater than oxide loss (increase in thickness).

( 0.0. =eompletely oxidized.

It will be obvious from the table that the hardness and rupture strength of the ternary-aluminum-chromium-iron alloy were considerably improved by the addition of niobium and/or tantalum. The oxidation resistance in most instances was not substantially decreased, and in some cases it was even improved.

It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

What is claimed is:

1. A jacket for a fuel element of a neutronic reactor consisting 'of an alloy consisting of from 15 to by weight of chromium, from 5 to 6.5% by Weight of aluminum, atotal of from 2.5 to 10% by weight of at least one metal selected from the group consisting of niobium and tantalum, said niobium content being within the range of from 2.5 to 10% and said tantalum content being Within the range of from 5 to 10% and the balance of said alloy being iron.

2. A jacket for a fuel element of a neutronic reactor consisting of an alloy consisting of 25% byweight of chromium, 5% by Weight of aluminum, a total of from References Cited in the file of this patent UNITED STATES PATENTS 2,192,742 Howe Mar. 5, 1940 2,210,309 Swinden Aug. 6, 1940 2,703,355 Hagglund Mar. 1, 1955 2,708,656 Fermi et a1 May 17, 1955 OTHER REFERENCES The Iron Age, March 22, 1951, pages -69. 

1. A JACKET FOR FUEL ELEMENTS OF A NETRONIC REACTOR CONSISTING OF AN ALLOY CONSISTING OF FROM 15 TO 30% BY WEIGHT OF CHROMOUM, FROM 5 TO 6.5% BY WEIGHT OF ALUMINUM, A TOTAL OF FROM 2.5 TO 10% BY WEIGHT OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISSTING OF NIOBIUM AND TANTALUM, SAID NIOBIUM CONTENT BEING WITHIN THE RANGE OF FROM 2.5 TO 10% AND SAID TANTALUM CONTENT BEING WITHIN THE RANGE OF FROM 5 TO 10% AND THE BALANCE OF SAID ALLOY BEING IRON. 